As is well known in the art, Rubidium (82Rb) is used as a positron emission tomography (PET) tracer for non-invasive measurement of myocardial perfusion (blood flow).
Recent improvements in PET technology have introduced 3-dimensional positron emission tomography (3D PET). Although 3D PET technology may permit more efficient diagnosis and prognosis in patients with suspected coronary artery disease, the sensitivity of 3D PET requires very accurate control of the delivery of 82Rb activity to a patient being assessed.
FIGS. 1 and 2 illustrate a conventional rubidium elution system used for myocardial perfusion imaging. As may be seen in FIG. 1, the elution system comprises a reservoir of sterile saline solution (e.g. 0.9% Sodium Chloride Injection), a pump, and a strontium-rubidium (82Sr/82Rb) generator. In operation, the pump causes the saline solution to flow from the reservoir 4 and through the generator 8 to elute the 82Rb. The active solution output from the generator 8 is then supplied to a patient (not shown) via a patient outlet 10.
When the system 2 is not in use, the amount of 82Rb within the generator 8 accumulates until a balance is reached between the rate of 82Rb production (that is, 82Sr decay) and the rate of 82Rb decay. As a result, the 82Rb activity level in the active saline emerging from the generator 8 tends to follow a “bolus” profile 12 shown by the solid line in FIG. 2a. In particular, at the start of an 82Rb elution “run”, the activity level rises rapidly and peaks, as accumulated 82Rb is flushed out of the generator 8. Thereafter, the activity level drops back to a substantially constant value. The maximum activity level Amax (bolus peak) obtained during the run is dependent on the amount of accumulated 82Rb in the generator 8, and thus is generally a function of the system's recent usage history, principally: the current 82Rb production rate; the amount of accumulated 82Rb (if any) remaining at the end of the previous elution run; and the idle time since the previous run. The generally constant level of the bolus tail is dependent on the rate of 82Rb production and the saline flow rate produced by the pump 6.
As is well known in the art, 82Rb is generated by radioactive decay of the 82Sr, and thus the rate of 82Rb production at any particular time is a function of the mass of remaining 82Sr. As will be appreciated, this value will diminish (exponentially) through the useful life of the generator 8. The result is a family of bolus curves, illustrated by the dashed lines of FIG. 2a, mapping the change in elution system performance over the useful life of the generator 8.
Because of the high activity level of 82Rb possible in the generator 8, it is desirable to limit the total activity dosage delivered to the patient during any given elution run. The total elution time required to reach this maximum permissible dose (for any given flow rate) will therefore vary over the life of the 82Sr charge in the generator 8, as may be seen in FIG. 2b, where the total activity dose, represented by the area under each curve, is equal in both cases.
A limitation of this approach, particularly for 3D PET imaging, is that the delivery of a high activity rate over a short period of time tends to degrade image quality. Low activity rates supplied over a relatively extended period are preferred. As a result, the user is required to estimate the saline flow rate that will obtain the best possible image quality, given the age of the generator and its recent usage history, both of which will affect the bolus peak and tail levels. This estimate must be continuously adjusted throughout the life of the generator 8, as the 82Sr decays.
Accordingly, techniques for controlling an 82Rb elution system that enable a desired activity level to be supplied over a desired period of time, independently of a state of the 82Sr/82Rb generator, remain highly desirable.